The rotor shaft in a steam turbine power system or the like is commonly journaled in bearing systems wherein oil or a like viscous film is disposed between the bearing structure and the outer surface of the rotor shaft to reduce frictional wear in the journal area. Early journal bearing systems comprised a sleeve type bearing wherein the journal was contained in a sleeve structure with an oil film disposed between the journal and the bearing sleeve.
However, several problems became known in the art of these oil bearing systems. As known in the art, the vibrations of the rotor shaft are dependent upon the characteristics of the viscous film layer within which the shaft rotates. Upon rotation of the shaft within the viscous layer, the viscous forces applied to the shaft from the film can be analyzed based upon well known spring and damper properties. As is also known, the viscous layer dampens excessive vibrations of the rotating body.
Operation of the rotor system with sleeve type bearings was known to create an undesirable cross coupling of horizontal and vertical forces on the journal. When the rotational speed of the rotor is operated at a certain "critical speed" associated with natural frequency of the system, this cross coupling of forces on the shaft was known to create undesirably excessive vibration levels, resulting in the so called "oil whirl" problem. The oil whirl problem could not be obviated in the known sleeve type bearing systems because of the cross coupled forces they develop. Consequently, in these systems the rotor may not be operated at desirable rotational speeds.
In response to the oil whirl problems associated with sleeve type bearing systems, two pad tilting pad bearing systems were developed wherein two pads are positioned at the bottom half of the bearing in a tilted relation for supporting the journal. The top half of the bearing remains of the sleeve type for containing the journal. In this regard, the tilting pad bearing system obviates the oil whirl problem due to the elimination of the above-mentioned cross coupled forces in a known manner. The known effect of the tilting pads is to direct the rotor shaft into a more desirable central location within the bearing structure.
However, although the two pad bearing provides stability in the rotor system by eliminating bearing cross-coupled properties, these bearing systems do not have the capability to move away criticals if they occur at running speed.
A known bearing system which provides some stability for the rotor system is commonly referred to as a pressure dam type bearing. This type of bearing system is similar to the sleeve type bearing structure, however, a step or "dam" is formed in a portion of the inner surface of the top half of the bearing, adjacent to the outer surface of the rotor shaft. Upon rotation of the rotor shaft, oil collects under pressure in the dam such that a downward force is directed on the rotor shaft to pre-load the journal, having the effect of damping the vibrations of the journal and maintaining the rotor shaft in a centralized position. This pre-loading of the journal may, under some circumstances, alter the natural frequency of the system so as to raise the rotational critical speed above the desired operational rotation speeds for the rotor shaft such that the turbine system can be operated without undesirably excessive vibrations in the journal area.
However, with these known pressure dam bearing systems, since the dimensions and location of the pressure dam is fixed, the angle at which the pressure dam force is exerted downwardly upon the rotor shaft is also fixed. Accordingly, control of the oil damping forces under the influence of the pre-loading force is limited to specific operating conditions.
It is therefore desirable to provide a journal bearing system which will effectively move rotor system criticals away from the operating range and maintain acceptable levels of vibration over a wide range of operating conditions.